ACCESSORY CHROMOSOME IN A FROG. 75 



At the side of the metaphase plate opposite from the X-body 

 in one of the three cells, another single body, evidently the 

 product of a precocious division, was observed. This is shown 

 in Fig. 23. In this cell, the dumb-bell accessory seems to have 

 divided, the halves passing to opposite poles. This explanation 

 may also hold for the other two cells in which the X-body 

 appears single, though the other half was not observed. There 

 is a possibility that the single appearance of the X may be due 

 to end views of the body, though such a possibility seems slight. 

 The very large size of the X would, in my opinion, preclude any 

 such possibility. 



The secondary spermatocytes, resulting from the division of 

 one of these three cells, the cell in which the X-body divided 

 (Fig. 23), would each receive thirteen chromosomes, each cell 

 receiving half of the accessory; whereas, in the case of the remain- 

 ing two cells, one half the second spermatocytes would receive 

 thirteen chromosomes, the other half, twelve. 



Three other cases of dividing first spermatocytes were ob- 

 served, in which the typical dumb-bell-shaped X-body had 

 migrated very early to one pole of the cell, and at the other pole 

 a large, round chromosome appeared. 



The two parts of the X-body in one cell, were unequal in size, 

 one being considerably smaller than the other. (See Figs. 24- 

 26.) The single, round chromosomes at the opposite pole from 

 the X-body in Fig. 24, judging from its size and shape, appears 

 to be the true half of the accessory or X. 



The behavior of the chromosomes in these three cells is difficult 

 to explain satisfactorily. It is obvious, however, that the second- 

 ary spermatocytes resulting from such divisions would receive 

 unequal amounts of chromatin. 



Repeated examination of my material convinces me that the 

 normal distribution of chromatin to the secondary spermatocytes, 

 at the first maturation division, is twelve chromosomes, plus the 

 dumb-bell-shaped X to one cell, and twelve ordinary chromo- 

 somes to the other. 



In regard to the case of those cells just described in which the 

 X-body appears to have divided precociously, half passing to 

 each pole in advance of the other chromosomes, it is probable 



